With growing concerns about global environmental issues and energy issues, solar batteries have been attracting attention as a clean, sustainable means of energy generation. For outdoor application such as use on the roofs of buildings, the solar batteries are generally used in the form of solar cell modules.
A solar cell module is generally produced by the following procedures: first, for example, either crystalline solar cells made of poly- or mono-crystalline silicon or thin-film solar cells obtained by depositing on substrate (e.g., glass substrate) very thin films of the order of several micrometers in thickness (e.g., amorphous silicon or crystalline silicon films) are produced. Next, in the case of a crystalline solar cell module, a module assembly is constructed in which a solar cell module protective sheet (surface-protective sheet), a light-receiving-side solar cell encapsulation sheet, the crystalline solar cells, a back-side solar cell encapsulation sheet, and a solar cell module protective sheet (rear protective sheet) are sequentially laminated on top of each other. On the other hand, in the case of a thin-film solar cell module, a module assembly is constructed in which the thin-film solar cells, a solar cell encapsulation sheet, and a solar cell module protective sheet (rear protective sheet) are sequentially laminated on top each other.
A solar cell module is then manufactured for example by a vacuum lamination process in which the module layers are laminated together under heat and pressure. Solar cell modules manufactured in this manner exhibit weather resistance, lending themselves to be suitably used in outdoor environment such as on the roofs of buildings.
Such a solar cell module is exposed to sunlight and the elements for a long period and therefore required to have excellent durability. In order to improve the durability of encapsulation sheets, a resin material used is generally mixed with additives such as ultraviolet absorbers, antioxidants, or hindered amine light stabilizers (see, e.g., Patent Literatures 1 and 2).
For improved light use efficiency of solar cells, a solar cell has been developed with enhanced external quantum efficiency for ultraviolet light having a wavelength of 400 nm or 360 nm or lower (see, e.g., Non-Patent Literature 1). In the case of using such a solar cell with enhanced external quantum efficiency for ultraviolet light, ultraviolet light having a wavelength of 360 nm or lower is absorbed by an encapsulation sheet containing an ultraviolet absorber, particularly when the encapsulation sheet is disposed on the light-receiving side. For this reason, ultraviolet light energy cannot be used.
On the other hand, in the absence of the ultraviolet absorber, ultraviolet ray contained in sunlight reaches as far as the rear protective sheet to cause degradation or discoloration of the sheet. PTL 3 has proposed a technique of effectively using ultraviolet light and preventing the discoloration of the rear protective sheet with a configuration wherein the light-receiving-side encapsulation sheet contains a reduced amount of ultraviolet absorber so that the back-side encapsulation sheet contains a higher concentration of ultraviolet absorber than the light-receiving side encapsulation sheet.